The posteroventral cochlear nucleus (PVCN) has an integral role in hearing: it is a major component of the efferent feedback pathway to the cochlea; it is part of the binaural processing system within the hindbrain; and it is part of the monaural ascending auditory pathway to the midbrain. It is the integrative function of the PVCN in both the ascending and descending auditory pathways and the fact that PVCN is an obligatory synaptic station of the auditory nerve, that establishes its importance in hearing, both centrally and peripherally. Although it is such an essential ingredient, except for ongoing single-cell studies comparing physiological response property with cellular morphology, study of the PVCN has been lacking. In an effort to correct this deficiency and to complement the single-cell studies, the broad long-term objectives of this study are to discover and describe the neuroanatomical and neurochemical structure of the mammalian PVCN. This will be accomplished with a multi-faceted approach using for example, tract-tracing studies of PVCN projections and connections and immunohistochemical and receptor binding studies of PVCN transmitters and their receptors. To test the hypothesis that the projection pattern of each of the three major PVCN cellular regions is different and that, within target nuclei, the neurons of each PVCN cellular region make connections with a unique set of target neurons, we propose four specific aims: 1) define all central nuclear targets of PVCN; 2) define specific nuclear targets of each of the three major cellular regions of PVCN; 3) determine whether descending auditory neurons projecting to the cochlea are specific targets of any of the three major PVCN cellular regions; and 4) determine whether auditory neurons projecting to the inferior colliculus are presumptive targets of any of the three major PVCN cellular regions. The specific aims will be accomplished with modem neuroanatomical techniques including Phaseolus vulgaris-leucoagglutinin and protein-gold tract-tracing, neurotransmitter immunohistochemistry, and electron microscopy. The proposed study will establish the anatomy of the PVCN in terms of its central projections and connections with specific target neurons that, in turn, project to either the cochlea or midbrain. Defining the central connections with the efferent system will aid in the understanding of the role of the efferent system itself i.e., the detection of signals in a noisy environment, protection of the ear from damage by loud sound, or generation of otoacoustic emissions. Defining the connections to the hindbrain and midbrain will reveal the substrates Utilized for sound localization and perception of auditory space. Moreover, the structural mechanisms that underlie the representation and processing of auditory signals will be defined, leading to a better understanding of the central mechanisms of complex sound perception that may expedite development of a central prosthetic devise used to treat profoundly deaf persons.